The thin-film-transistor liquid crystal display (TFT-LCD) industry has now dominated the flat panel display market. Generally, in a typical liquid crystal display (LCD), the liquid crystal layer is disposed between two glass substrates and the plural pixel units with switching thin-film-transistors (TFTs) are formed on the inner surface of one substrate to provide driving voltages. The liquid crystal (LC) cell together with the polarizers functions like a light valve to modulate amplitude for different grey levels. The LCD technology is fairly mature as the issues of viewing angle, color gamut, power consumption, and contrast ratio have been improved a lot. The response time is been improved to 2-5 ms (mini seconds) by using low viscosity LC materials and thinner cell gap fabrication techniques. However, to reduce the motion image blur, a faster molecular response is needed.
Recently, a blue-phase liquid crystal (BPLC) has been developed; it has a fast response time within submillisecond. In comparison to conventional nematic LCDs, the blue-phase LCD exhibits at least four features: 1) it does not require any alignment layer, such as polyimide, which not only simplifies the manufacturing processes but also reduces the cost; 2) its response time is in the submillisecond range, which helps to minimize the motion-image blur; 3) the dark state of the blue-phase LCD is optically isotropic so that its viewing angle is wide and symmetric, in which optical compensation films may or may not be needed, depending on the actual applications; 4) in an in-plane switching (IPS) like structure, the transmittance is insensitive to the cell gap, as long as the cell gap exceeds 2-3 μm depending on the birefringence of the LC composite employed. This cell-gap insensitivity is particularly attractive for fabricating large-panel or single-substrate LCDs, in which cell-gap uniformity is a big concern.
Blue-phase liquid crystal exists in a very narrow temperature range (1˜2° C.) between chiral nematic and isotropic phase; its molecular structure is comprised of double-twisted cylinders arranged in a cubic lattice with periods of ˜100 nm [Meiboom, et al., Theory of the blue phase of cholesteric liquid crystals, Phys. Rev. Lett. 46, 1216-1219 (1981)]. Blue-phase liquid crystals have been studied for several decades; however their mesogenic temperature range is too narrow for any practical applications. With the polymer-stabilization method, the mesogenic temperature range of blue phase can be widened, covering the room temperature [H. Kikuchi, et al., Polymer-stabilized liquid crystal blue phases, Nature Materials 1, 64-68 (2002)]. In the voltage-off state, blue-phase liquid crystal appears optically isotropic. As the voltage increases, based on Kerr effect, the LC refractive index distribution becomes anisotropic. The induced birefringence Δn is proportional to the incident wavelength λ, the Kerr constant K, and the square of the applied electric field E as Δn=λKE2. The induced Δn appears to change the blue-phase liquid crystal from the sphere into an ellipsoid like a uniaxial medium where the optic axis is along the electric field vector. In the IPS based electrode structure, the electric field in the direction parallel to the substrate is preferred so that the induced birefringence Δn will be along the horizontal direction that is parallel to the polarizer surface plane. For a transmissive display, when the device is interposed between two crossed polarizers, the transmittance increases gradually as the voltage increases. [Z. Ge, et al. Electro-optics of polymer-stabilized blue phase liquid crystal displays, Appl. Phys. Lett. 94, 101104 (2009); L. Rao, et al., Emerging liquid crystal displays based on the Kerr effect, Mol. Cryst. Liq. Cryst., 527, 186-198 (2010)]. Generally, for a uniaxial medium placed between two crossed linear polarizers, the output transmission is T=T0 sin(2φ)2 sin(δ/2)2, where φ is the angle between optic axis of the uniaxial media and the absorption axis of the polarizer, δ=2πdΔn/λ is the phase retardation of the uniaxial medium. Thus, φ needs to be 45° away from the linear polarizer's absorption axis. In other words, the induced birefringence Δn also needs to be along that direction to have maximum transmittance. Besides, the induced birefringence Δn should also be parallel to the substrate surface to have a maximum δ. For vertical field switching, to gain the phase retardation, the incident light direction should not be parallel to the field direction. [H. C. Cheng, J. Yan, T. Ishinabe, and S. T. Wu, Vertical field switching for blue-phase liquid crystal devices, Appl. Phys. Lett. 98, 261102 (2011).] Aside from blue-phase liquid crystal, other types of optically isotropic liquid crystal, e.g. polymer stabilized isotropic phase (PSIP) liquid crystal, is reported with similar electro-optic properties. [Y. Haseba, et al. Large electro-optic Kerr effect in nanostructured chiral liquid-crystal composites over a wide temperature range, Adv. Mater. 17, 2311 (2005)].
For blue-phase liquid crystals, or more generally the optically isotropic liquid crystals, the Kerr constant K is wavelength dependent. The Kerr constant decreases with the increment of the wavelength. [L. Rao, et al., “Emerging liquid crystal displays based on the Kerr effect”, Mol. Cryst. Liq. Cryst., 527, 186-198 (2010); M. Jiao, et al., “Dispersion relation on the Kerr constant of a polymer-stabilized optically isotropic liquid crystal,” Phys. Rev. E 83, 041706 (2011).] Therefore, the voltage-transmittance curves for red, blue and green three colors will not overlap.
A known IPS electrode driven BPLCD is shown in FIG. 1A, where a blue-phase liquid crystal layer 12 is inserted between two glass substrates 10a and 10b, and a pixel electrode 11a and a common electrode 11b are both formed on the bottom substrate 10a. Typically, the polarizers 14a and 14b interpose the substrates 10a and 10b and the blue-phase liquid crystal layer 12 therebetween. The electrodes 11a and 11b are made of metal or indium tin oxide (ITO) in a stripe shape, and the stripe width w is around 3 to 10 μm and the spacing 1 between two electrodes 11a and 11b is about 6 to 20 μm. In the related art, the thickness t of the metal or ITO electrodes 11a and 11b in the stripe shape are typically below 150 nm. And this low stripe electrode height confines the strong electric fields (as shown by the dash lines 13 in FIG. 1A) to mainly distribute near the bottom substrate surface. As shown in FIG. 1B, the dispersion for V-T (voltage versus normalized transmittance) curves R, G and B of red, green and blue three colors is quite severe, in which the IPS structure here has an electrode width w of 2 μm and gap 1 of 5 μm.